The present invention relates to a nitrogen dioxide gas sensing method, a nitrogen dioxide gas sensor element, and a nitrogen dioxide gas sensor using the same.
Currently, the influence of SO2 and NOx on the environment poses a problem. SO2 and NOx are produced by combustion of fossil fuels and cause acid rain and photo-chemical smog. In Japan, environmental standards are set for these pollutants, e.g., the concentration of atmospheric NO2. Gas concentration measurement is performed by an automatic measurement method in regular monitoring stations at different locations. As an environmental standard, the average value per hour in a day is approximately 60 ppb or less.
Although these gas concentration meters can measure a few ppb, i.e., a slight amount of gas, they are expensive and require maintenance. Also, automatic measurement requires an enormous cost for electricity and the like, and a power supply and an installation place must be secured for the measurement. However, to accurately investigate gas concentration distributions and evaluate terrestrial environmental influence, it is necessary to increase the number of monitoring locations and perform nationwide environmental monitoring. For this purpose, the cumulative use of inexpensive, small, and easy-to-use gas sensors or simple measurement methods (or monitoring devices) is possible.
Presently, the development of a semiconductor gas sensor, a solid-electrolyte gas sensor, an electrochemical gas sensor, and a quartz oscillation gas sensor is being extensively made. However, these sensors are developed to evaluate response in a short time period; i.e., only few sensors are developed for monitoring requiring data accumulation. Also, since the sensitivity is about 1 ppm, these sensors cannot sense real environmental concentrations (e.g., about 10 ppb for NOx). Additionally, the influence of other gases is not negligible in many instances.
A method using a sensor tube gas meter is also developed to perform short-time measurement on the spot but is difficult to use in cumulative measurement. Furthermore, an operator must go to the measurement site, and personal errors occur in reading colors. In many instances, interference of other gases or disturbance is a problem.
A common simple measurement method is to receive air by using a pump, collect NOx gas by directly collecting it into a sampling bag (a direct collection method), by using a solid-state adsorbent (a solid collection method), or by collecting it into an absorbing solution (a liquid collection method), and analyze the collected gas by gas chromatography. In any of these methods, however, it is necessary to transport not only samples but also peripheral apparatuses such as a pump. In the direct collection method, it is difficult to store gas because the size of the sampling bag is limited. The solid collection method and the liquid collection method require a process of sensing the collected gas.
As a simple monitoring method requiring no suction, the use of a passive sampler for environmental measurement has attracted attention. Examples of an NOx passive sampler are a nitration plate method and a triethanolamine (TEA) batch method. The effects of the wind speed, temperature, and humidity are examined to examine the quantitativeness. A general approach of measurement after sampling is to clean the sample to form a solution and analyze the solution by ion chromatography or absorptiometric analysis. Unfortunately, these samplers require cumbersome processes such as cleaning after collection and before analysis.
As a method of solving these problems, an NOx gas sensing method has been proposed. In this method, NO2 gas is reacted with a sensing reagent adsorbed in pores of a porous body as a transparent matrix adsorbent, and the transmitted UV-visible absorption spectrum is measured by a spectrophotometer to sense the amount of NO2. However, a side reaction occurs in this sensing method because a highly reactive material is used as the sensing reagent. Consequently, the sensitivity of the reaction product with NO2 as a target substance does not increase in the UV-visible absorption spectrum. Additionally, the side reaction product disturbs the UV-visible absorption spectrum to make high accuracy impossible to obtain.
In fine, the conventional methods require large expensive apparatuses to accurately sense nitrogen dioxide gas of the order of ppb in accordance with the environmental standards. Also, these methods are too cumbersome to allow easy detection of nitrogen dioxide gas.
It is, therefore, a principal object of the present invention to simply and accurately sense nitrogen dioxide gas.
To achieve the above object, according to an aspect of the present invention, a mixture of a diazotizing reagent which reacts with nitrous ions to produce a diazo compound, a coupling reagent which couples with the diazo compound to produce an azo dye, and an acid is placed in pores of a transparent porous body to prepare a sensor element. The light transmittance of this sensor element is measured to calculate the first transmittance. The sensor element is then exposed to air to be measured for a predetermined time. After that, the light transmittance of the sensor element is measured to calculate the second transmittance. Nitrogen dioxide gas in the air to be measured is sensed in accordance with the difference between the first and second transmittances.
In this method with the above arrangement, when the sensor element is exposed to an atmosphere containing nitrogen dioxide gas, the diazotizing reagent diazotizes nitrous ions produced by nitrogen dioxide adsorbed to pores of the sensor element to produce a diazo compound. The coupling reagent couples with this diazo compound to produce an azo dye. Consequently, the sensor element colors to make a difference between the first and second transmittances. This allows detection of the nitrogen dioxide gas.
According to another aspect of the present invention, a nitrogen dioxide gas sensor element comprises a transparent porous body, a diazotizing reagent placed in pores of the porous body to react with nitrous ions to produce a diazo compound, a coupling reagent placed together with the diazotizing reagent in the pores of the porous body to couple with a diazo compound to produce an azo dye, and an acid placed together with the diazotizing reagent in the pores of the porous body.
When nitrogen dioxide gas enters the pores of the nitrogen dioxide gas sensor element with the above arrangement and is adsorbed to the pores, the diazotizing reagent reacts with nitrous ions produced by the nitrogen dioxide gas to produce a diazo compound. The coupling reagent couples with this diazo compound to produce an azo dye. Consequently, the nitrogen dioxide gas sensor element colors.
According to still another aspect of the present invention, a nitrogen dioxide gas sensor comprises a light-emitting unit for emitting light, a light-sensing unit having a light-receiving surface opposed to a light-emitting surface of the light-emitting unit to output an electrical signal corresponding to a light amount received by the light-receiving surface, a sensor element inserted between the light-emitting unit and the light-sensing unit, and an electric meter for measuring the state of the output electrical signal from the light-sensing unit. The sensor element comprises a transparent porous body, a diazotizing reagent placed in pores of the porous body to react with nitrous ions to produce a diazo compound, a coupling reagent placed together with the diazotizing reagent in the pores of the porous body to couple with the diazo compound to produce an azo dye, and an acid placed together with the diazotizing reagent in the pores of the porous body.
In the sensor with the above arrangement, when nitrogen dioxide gas enters the pores of the sensor element and is adsorbed to the pores, the diazotizing reagent reacts with nitrous ions produced by the nitrogen dioxide gas to produce a diazo compound. The coupling reagent couples with this diazo compound to produce an azo dye. Consequently, the nitrogen dioxide gas sensor element colors. Meanwhile, the light emitted from the light-emitting unit enters the light-sensing unit via the sensor element. The electric meter measures the color change of the sensor element as a change in the output electrical signal from the light-sensing unit.